Conventionally, a composite molded article of a thermoplastic elastomer and a thermoplastic rigid resin has been produced by separately preparing a molded product of an elastomer and a molded product of a thermoplastic rigid resin, and then boding them to each other by using a double-sided adhesive tape. However, the adhesion operation is troublesome.
JP-B-2-8572 (the term "JP-B" used herein means "an examined published Japanese patent application") discloses a method which does not require an adhesion operation. In the method, a propylene resin is melt-injected, and then an unvulcanized thermoplastic elastomer is melt-injected thereon to obtain a composite molded article. However, this molding process has problems in producing large molded articles due to the poor fluidity of thermoplastic elastomers, and it suffers problems such as generation of burrs or flashes at the gate during molding. Moreover, in the molding process, there are unsolved problems of warpage deformation on the polypropylene resin portion depending on the shape of the molded article, and that fusion does not occur between the polypropylene resin and the unvulcanized thermoplastic elastomer. Furthermore, in general, an unvulcanized thermoplastic elastomer (flexible material) yields a larger mold shrinkage factor as compared with a polypropylene resin (rigid material). Thus, when the unvulcanized thermoplastic elastomer is molded after molding the polypropylene resin, this difference is further increased due to the time difference. Accordingly, the deformation of the molded article and the generation of internal stress have been considered as problems to be solved. Improvement of a mold structure also must be studied to solve the problem that an undercut of the thermoplastic elastomer part cannot be formed at the joint portion between the thermoplastic rigid resin and the thermoplastic elastomer.
JP-B-6-75927 discloses a method for molding a tank made of resins. In the method, a sealing material made of a thermpolastic and elastic material is injected to form a sealing part, and then a thermoplastic resin is injected to form a thermoplastic resin part. However, further improvements are required to make the resin tank lighter and to increase the adhesion strength between the sealing material and the thermoplastic resin.